The humidification measurement system designed in laboratory was used to measure relative humidity and temperature of reaction gases passing through internal or external humidifier which was used in proton exchange membrane fuel cell test station. The relative humidity of gases was stabilized after $10~20$ minutes and thus credibility of data could be assured. The effect of relative humidity on fuel cell performance could be analyzed by humidity measurement system. Extreme caution was needed to avoid humidity sensor mal-function or failure which is probable in experiment of high humidity condition near $100\%$. The amount of water carried by gas through humidifier was increased along the flow rate of gas. However, the extent of increase was lowered at high gas flow rate. These phenomena could be analyzed as residence time effect of gas in humidifier.